Rilling and Houde; Variability in growth and mortality of Anchoa mitchtlli 



561 



in the 3-12 mm SL size range during July. The in- 

 crease in gi-owth and decHne in mortahty rates be- 

 tween June and July accounted for the drops in Ml 

 G ratio. On average, larval cohorts lost biomass in 

 June (M/G>1.0), but gained biomass in July [M / G 

 <1.0). In June, cohorts at 12 mm SL supported only 

 6.6% of the biomass present at 3 mm SL, whereas, 

 in July, cohorts at 12 mm SL supported 457.19^ of 

 their 3-mm-SL biomass. Each regional MIG ratio 

 also declined between June and July. 



Predicted abundance of larval daily cohorts at 12 

 mm SL, based upon the MIG ratios and estimated 

 regional abundance-at-age data, was highest in the 

 upper bay in June but shifted to the mid and lower 

 bay in July (Table 2). Stage-specific mortality rates, 

 estimated from declines in abundances, were high- 

 est for the youngest stages, and declined with increas- 

 ing age (Table 3). In this analysis, abundances were 

 estimated for cohorts at 18 days after hatching, when 

 regional mean lengths ranged from 12.4 to 16.4 mm 

 SL. Despite highest regional mortality, daily cohorts 

 from the lower bay in July produced the most 18- 

 day-old larvae (1.5 x 10*). The daily production of 



18-day-old larvae in the lower bay was 3.4-4.6 times 

 higher than in other regions in July, and from 7 to 

 50 times higher than in other regions in June. 



Cumulative mortalities from egg to 18-day old lar- 

 val stage were lowest in the mid bay in June but 

 lowest in the upper bay in July. Interestingly, those 

 regions had experienced the greatest egg to 3-day- 

 old larval mortalities, which then were followed by 

 low mortality in older larvae (Table 3). 



Predators and prey 



There was a significant between-cruises difference 

 in mean combined biovolumes of two common gelati- 

 nous predators of bay anchovy eggs and larvae, the 

 ctenophore Mnemiopsis leidyi, and the scyphomedusa 

 Chrysaora quinquecirrha, (t-test,P<0.00l) (Table 1). 

 Mean biovolumes of the ctenophore shifted region- 

 ally and declined by a factor of four in July. The 

 scyphomedusan did not occur in June and had a mean 

 biovolume of 29.5 niL/ni" in July, and there was no 

 indication of regional differences (AN OVA P>0.05) 

 (Table 1). 



